Interventions to increase aerobic exercise in patients with obsessive compulsive disorder (OCD) represent a potentially useful yet unexplored strategy for ameliorating OCD symptomatology. There are currently no established exercise interventions for use with this population. The long-term objectives of this research program are to improve treatment options for persons with OCD, by: 1) establishing the efficacy of an aerobic exercise intervention for individuals with OCD, and 2) advancing knowledge of the relationship between aerobic exercise and OCD symptomatology. In a pilot study conducted by our research group, individuals (n=15) who had clinically significant residual symptoms despite receiving treatment for OCD experienced significant and clinically meaningful decreases in OCD symptoms following participation in a 12-week aerobic exercise intervention. Treatment gains were largely maintained over a 6-month follow-up. However, due to the lack of a control condition in this study, the treatment gains could not be attributed directly to the exercise intervention. This project represents a collaboration among researchers with considerable experience in both the treatment of OCD and the application of exercise-based interventions. One hundred and two (102) patients with OCD with clinically significant OCD symptoms despite engaging in recommended treatment (pharmacotherapy or CBT) will be randomly assigned to receive either a 12-week moderate intensity aerobic exercise (AE) intervention or a health education control (HEC) comparison intervention. Follow-up interviews will be conducted at the end of treatment and at 3-, 6- and 12- month follow-ups. Based on the outcome of this preliminary trial, the exercise intervention will be further refined and readied for larger-scale clinical trials.